We need time to indicate when we need to get up, go to work or to an appointment. This is our social time, the time you see on your watch or clock. We can adjust this social time easily to the time we want it to be, but that doesn’t affect our bodies, for example when we feel tired or energized. Our bodies have an internal clock, a biological clock, to indicate the time of the day. This clock needs light to synchronize with the natural light-dark cycle. The sun is at it’s highest at 12.00, this is the time you feel most energized and around twilight hours you begin to feel more tired, this is because your internal clock signals to your body it’s going to be dark soon and needs to produce the sleeping hormone melatonin. This is the function of our biological clock, to anticipate the predictive 24-hour pattern of our environment.
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What if there is no light? This is the situation when you are inside all day. The light levels indoors are too low during daytime and too high in the evening to set our biological clock effectively. Your sleep/wake cycle would go from 24 hours to a 24 hours and about 15 minutes cycle. Your body can’t synchronize with the environment; the timing of when you are actually ready for sleep shifts day by day. Scientists compare this with living with a permanent jetlag and you could risk depression and insomnia.